On the Airbus A300-B4 the end of the fuel jettison pipe is rubber. It is called "Anti Corona", and is there to ensure no static discharge when dumping fuel.
I suspect the VC-10 pipe is a mod.
I remember the 1-11 fuel jettison pipe on the -400 srs. and it was metal.
We had an ex Philipines Airlines one back at Hurn about 1972, and when we tested the system not much fuel came out on the starboard side. On investigation, a dead green snake was found inside the pipe on top of the wing!!
Also this A/C had many cockroaches and other nasties within. Even after fumigation some things were still alive.
Fun times.